Despite enormous scientific effort, the development of a vaccine against HIV has proven to be a largely elusive goal. There are several major factors complicating the creation of such vaccine.
One problem stems from a very low immunogenicity of the viral surface. Pairs of the envelope spike proteins (gp120 and gp41) form a trimer, inside of which much of the potentially antigenic surface of the unprocessed precursor protein (gp160) is buried. Moreover, the “outer” face of gp120 is extensively glycosylated (and therefore unavailable for peptide—recognizing antibodies), further complicating the problem.
Secondly, the mature envelope oligomer is itself a very weak antigen. Many explanations have been proposed to explain the unusually low antigenicity of the viral envelope spikes. The “glycan shield” concept implies that steric hindrance created by N-linked carbohydrates of gp120 prevents the immune system form generating antibodies with a broadly neutralizing action. Another hypothesis states that binding of neutralizing antibodies to the CD4 site of gp120 leads to conformational changes and is entropically disfavored, thereby allowing for HIV neutralization escape. It has also been suggested that a very strong initial immune response to gp160, which does not lead to broadly neutralizing antibody production (vide supra) suppresses response to the mature oligomer, which is expressed in much lower concentrations.
In addition, extremely high degree and rate of viral variation provide a powerful mechanism for HIV to escape immune defense.
Accordingly, commonly utilized vaccine formulations have been unable to elicit a potent and broadly neutralizing antibody response. Administration of the whole virus in attenuated or inactivated form presents safety issues as well as the problem of low antigenicity. Immunization with a part of HIV DNA in a carrier is more promising, however it requires a very careful choice of the carrier virus. Also, low envelope antigenicity still remains a serious obstacle to the success of this method. A solution may lie in the use of artificial HIV antigens based on the epitopes of known broadly neutralizing antibodies. A highly focused immune response may be developed with this approach, potentially circumventing the problem of low antigenicity. The biggest challenge in this case is the design and synthesis of the antigens.
Gp120 surface carbohydrates can be seen as an attractive target for such design. There are a number of molecules that can efficiently bind to HIV envelope glycans. Among them, the dendritic cell receptor DC-SIGN has been demonstrated to recognize the internal tri-mannose segment of the N-linked oligosaccharides. A bacterial protein cyanovirin-N efficiently binds high-mannose type gp120 carbohydrates. Also, one of the most potent broadly neutralizing antibodies known to date, the 2g12, has been shown to have a carbohydrate epitope. Administering synthetic antigens containing one or more glycans on a part of gp120 peptide backbone or appropriately chosen linker system and further conjugated to an antigenic carrier could elicit strong immune response ultimately aimed at the real viral envelope. Some of the N-linked carbohydrates of gp120 appear to be conserved in most of HIV primary isolates. Since the glycans recognized by these molecules are located on the outer, “silent” face of the oligomer, they are easily accessible for antibody binding. Entropically disfavored interaction does not present a problem since the epitope does not overlap with the CD4-binding site. Finally, an extensive glycosylation of the envelope is an advantage, rather than a problem for such antigen design.
Accordingly, there remains a need for novel synthetic methods leading to the preparation of gp120 glycans and conjugates thereof, and their evaluation in immunologic and therapeutic studies.